(define (f n)
    (cond ((<= n 3) n)
          (else (+ (f (- n 1)) (f (- n 2)) (f (- n 3))))))

(define (f2 n)
    (f-iter 3 2 1 n))

"a -- F(n-1)
 b -- F(n-2)
 c -- F(n-3)
"
(define (f-iter a b c count)
    (cond ((= count 1) c)
          ((= count 2) b)
          ((= count 3) a)
          (else (+ (f-iter (+ a b c) a b (- count 1))))))
